This invention relates to a plasma torch of the type in which the electrode and the hood are brought into contact with one another to ignite the arc, and which features improved electrode air-cooling devices.
In the open position the electrode comes to rest against the torch body, thus determining the extent of its travel and therefore its working position.
In particular, in the torch in accordance with the invention, the electrode is hollow and contains a tube which directs a jet of pressurised air towards the tip of the electrode to cool it.
The same air is then directly partly towards the outer surface of the hood to complete cooling, and partly towards the plasma chamber for cutting.
As a result of the special configuration of the parts, a torch can be made which is highly compact but still guarantees precision of movement and efficient cooling of the various components, especially at the tip of the electrode where heating is greatest.
Plasma torches in which the electrode is air-cooled are already known; these are mainly low-powered torches.
In these known torches, a quantity of air is sent to the plasma chamber for cutting, and the same air is also used to cool the outside of the electrode.
This is a system of limited efficacy, because the amount of air used for cutting is small (approx. 10-20 l/min.) and only the outside of the electrode is cooled.
Higher-powered plasma torches are also known in which cooling is effected by circulation of a flow of water conveyed inside and outside the electrode.
As a result of French patent 2669846 by the same applicant, a plasma torch is also known in which an electrode is fitted to a support piston so as to constitute an assembly which moves inside the torch from a position in which the electrode rests against the wall of the plasma chamber to a working position in which the piston retracts, raising the electrode and moving it to a pre-set distance from the wall of the chamber.
This type of torch presents two drawbacks.
Since the piston moves to the end of its travel in the electrode raised position (in the case in point until it comes into contact with the upper wall of the cylinder) and the working position of the electrode is therefore determined by that of the piston, it follows that if the electrode is to be positioned at a specific working height (a characteristic which is crucial to the correct operation of the torch), it must be fitted with equal precision to the piston, with all the difficulties that involves.